Credential Devices (Key Cards)

A credential device identifies a person having legitimate authority to enter a controlled area. A coded credential (plastic card or key) contains a prerecorded, machine-readable code. An electric signal unlocks the door if the prerecorded code matches the code stored in the system. Credential devices only authenticate the credential; they presume that a user with an acceptable credential is authorized to enter. Various technologies are used to store the code or in a card.

With increasing frequency, credential devices are including color photographs and other authorizing symbols and authenticators. These devices are often referred to as employee identification cards. The most commonly used types of credential devices include magnetic-stripe cards, Wiegand-effect cards, proximity cards, optical-memory or laser cards, smart cards, and bar-code cards.

A magnetic-stripe card has a strip of magnetic material along one edge of the card that is encoded with data (sometimes encrypted). The data are read by moving the card past a magnetic read head. The data can be erased if the card is exposed to a magnet.

The Wiegand-effect card contains a series of thin, parallel wires about one-half inch long, embedded in the bottom half of the card. The wires are manufactured from magnetic materials that produce a sharp change in magnetic flux when they are exposed to a slowly changing magnetic field. This type of card is impervious to accidental erasure. The card reader contains a small read head and a tiny magnet to supply the magnetic field. It usually does not require external power.

A proximity card is not physically inserted into a reader; the coded pattern on the card is sensed when it is brought within several inches of the reader. Several techniques are used to code cards. One technique uses a number of electrically tuned circuits embedded in the card. Data are encoded by varying the resonant frequencies of the tuned circuits. The reader contains a transmitter that continually sweeps through a specified range of frequencies and a receiver that senses the pattern of resonant frequencies in the card.

Another technique uses an integrated circuit embedded in the card to generate a code that can be magnetically or electrostatically coupled to the reader. The power required to activate embedded circuitry can be provided by a small battery embedded in the card or by magnetically coupling power from the reader.

The optical memory card, commonly called the laser card, uses the same technology developed for recording CDs. Data are recorded on the card when a laser burns a microscopic hole in a thin film covering the card. A laser senses the holes when the data are read. The typical laser card can hold several megabytes of user data.

A smart card is embedded with a microprocessor, memory, communication circuitry, and a battery. The card contains edge contacts that enable a reader to communicate with the microprocessor. A vast array of data, including entry-control information, may be stored in the microprocessor's memory.

A bar code consists of black bars printed on white paper or tape that can be easily read with an optical scanner. This type of coding is not widely used for entry control because it can be easily duplicated. It is possible to conceal the code by applying an opaque mask over it. In this approach, an infrared scanner interprets the printed code. For low-level security areas, bar codes can provide a cost-effective means of entry control. Coded strips and opaque masks can be attached to existing ID badges, alleviating the need to completely replace existing badges.